Cermet resistors, their composition and method of manufacture



Dec. 17, 1968 w. 'r. KELLY ETAL 3,4

CERMET RESISTORS, THEIR COMPOSITION AND METHOD OF MANUFACTURE Filed May 9. 1966 INVENTORS WILLIAM T. KELLY FIG. 3 LEON B. HORNBERGER MILTON J. STRIEF United States Patent ABSTRACT OF THE DISCLOSURE A cermet material and method of firing such material onto a nonconductive substrate to produce a resistance layer having high power stability. The material includes a mixture of finely divided particles of glass and particles of a filler material, such as ground steatite, alumina, calcium silicate, barium silicate and lead zirconate, which are mixed together with a noble metal in the form of a dissolved metal solution. The mixture is heated to drive off the volatile materials, mixed with a suitable liquid carrier, applied to the substrate and then heated to the fusion temperature of the glass but less than the melting temperature of the noble metal.

The present invention relates to improved cermet resistance elements and to their composition and method of manufacture.

Cermet resistance elements and compositions presently known in the art are exemplified by US. Patent No. 2,950,995 entitled, Electrical Resistance Element, US. Patent No. 2,950,996 entitled, Electrical Resistance Material and Method of Making Same, and US. Patent No. 3,149,002, Method of Making Electrical Resistance Element, all of which issued to Thomas M. Place, Sr., et al. and are assigned to Beckman Instruments, Inc., assignee of the present invention. The foregoing patents describe a resistance element formed of a layer of resistance material comprising a heterogeneous mixture of non-conducting binder material and minute conducting metal particles fired to a non-conducting base. In its most generally applied form, the non-conducting material is a ceramic, such as glass, and the layer is formed by heating a mixture of metal and glass particles at least to the melting point of the glass, so as to create a smooth-glassy phase with the metal particles distributed uniformly throughout the glassy-phase. Additional prior art describing cermet resistors and compositions are US. Patent No. 2,873,487 of Daniel E. Huttar, entitled, Resistor Enamel and Resistor Made Therefrom and US. Patent No. 2,924,540 of James B. DAndrea entitled, Ceramic Composition and Article.

Cermet-type resistance elements represent a substantial improvement over the majority of deposited layer type resistance materials in that they are extremely hard, have a smooth surface, and have utility over a wide range of different temperatures. Some cermet resistance films are, however, initially unstable when'an electrical power is applied thereto. That is, they tend to exhibit a notable change in resistance during the initial hours of electrical power operation. The resistance may change from one to five percent during the first -50 hours of power operation. After the first 25-50 hours of operation the rate of resistance change due to power application is vastly reduced and the resistors become more stable. For precise electrical circuits or systems this change in resistance may not be tolerable and cermet resistors for use in such systems may require an initial power stressing for a predetermined period before the resistor may be selected for such circuit or system.

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Another problem associated with cermet resistors and in the manufacture of resistance films, is the fact that the binding material or glass tends to bleed out around the edges of the deposited layer of material when the material is fired to a temperature sufficient to fuse the bindel material. The glass tends to flow or bleed out arounc' the edges and on the surface of the deposited shape producing a peripheral fringe area of high glass concentration. This tendency to bleed out sometimes creates 2 very high resistance connection when conductive terminations are added in the bleed out region.

Certain types of termination materials have fiuxing constitutents therein that sometimes react with the glass 01 metal components of the cermet resistance element when they are fired to produce a terminal connection. This produces a distorted and somewhat unstable connection. A reduction of the incompatibility between such terminatior materials and cermetmaterials is desirable.

Resistance elements and compositions, formed according to the present invention, have resulted in extremely power stable cermet resistors and have greatly reduced the bleed out tendency for the binder material for sucl: resistors. They also greatly resist the reaction that occurs with fluxing agents of termination materials and produce uniform and low resistance terminal connections.

Further advantages and features of the cermet resistors, formed according to the present invention, will become apparent as the description proceeds. A more thorough understanding of the invention may be obtained by study of the following detailed description taken in connectior with the accompanying drawing in which:

FIGURE 1 is a perspective view of one embodiment of the invention which is suitable for use in rotary potentiometers;

FIGURE 2 is a perspective of another embodiment oi the invention which is suitable for use in linear potentiometers as well as for fixed resistors; and

FIGURE 3 is a perspective view of a non-conductive substrate having formed thereon a resistor network of 2 type employed in micro-miniaturized electrical circuits.

In the structure of FIGURE 1, a layer 10 of cermet resistance material is fired to a non-conductive base mem ber 11. A pair of electrodes 12 and 13 are provided at each end of the layer 10 for connecting the fired cermet resistance element into an electrical circuit. This resistance element may be employed as a fixed resistor or ma) be combined with a rotating contact arm for use in 2 rotary rheostat or potentiometer. The base 11 may be 01 any suitable electrically non-conducting material whicl will withstand the elevated temperatures normally used in firing and fusing the glass or ceramic constituents o] the resistance material. Various ceramic materials are suitable for this use, those having a smooth, fine textured surface and being impervious to moisture and other liquids being preferred. Steatite, sintered or fused aluminas anc' zircon porcelains are examples of preferred materials fol forming the base member 11. While the particular composition of the base member does have some eifect on the resistance obtained, these effects are usually constant for any particular cermet material and the ultimate resistance to be obtained can be very closely predicted and maintained.

The electrically conductive electrodes 12 and 13 are conventional and may be formed by applying any of the well-known conductive metals, such as silver, goldpalladium alloys, or other metal pastes over or unde1 the layer of resistance material. These pastes are ther fired to convert the metallic paste into a thin layer oi metal which is firmly attached to the layer of resistance material.

FIGURE 2 illustrates another form of the resistance lement of the invention in which a layer 15 of resistance iaterial is applied to a rectangular base 16 and electrodes 17 and 18 are then added at the ends of the layer 15.

The enlarged view of a microminiature circuit element, illustrated in FIGURE 3, is formed of a suitable nonconducting substrate material such as alumina which has deposited thereon a number of fixed cermet resistance elements 21 adapted to be connected to other electrical components through conductive circuitry 22 also formed on the substrate and adapted to contact conductive electrodes 23 formed within the substrate.

It will be understood that the elements illustrated in FIGS. 1-3 are enlarged and that, in practice, the resistance layers formed on the substrate member are approximately .000] to .005 inch in thickness. Preferably, the thickness of the resistance layer is maintained at a constant value. A preferred thickness is approximately .001 inch and all of the resistors mentioned in the examples disclosed later in this specification were maintained at this thickness. The particular configurations of the resistance layers and substrate members can vary and are not limited to the arrangements disclosed in FIG- URES l3. These forms, however, are particularly suitable for use in fixed and variable resistance devices, such as potentiometers, and in microcircuit devices.

The aforementioned US. Patent No. 3,149,002 teaches a preferred method of preparing cermet resistance material and elements. The preferred method taught therein comprises, mixing finely divided glass particles with a metal alloy material in the form of a soluble noble-metal compound, such as a noble-metal resinate or organic compound of one or more of the noble metals. The glass binder, in the form of finely divided glass particles, is mixed or milled with a resinate, or other dissolved metal solution either organic or inorganic so that each glass particle is thoroughly wetted with noble-metal solution. This mixture is then gradually heated and constantly stirred to remove the volatiles and organic materials from the mixture and to decompose the noble metal compounds. The resulting dry material is ground to a fine powder and calcined for a short period to assure removal of all organic materials. The calcine is then ground to a fine powder, producing a dry material :onsisting of very small glass particles mixed with minute particles of noble metal most of which are believed to be physically attached to or coated onto the glass particles.

In order to form resistors from this resistance material, the dry powder is mixed with a suitable liquid :arrier to form a fluid composition which can then be applied in a thin layer to the base member. The fluid resistance composition may be applied in any suitable manner such as by brushing, spraying, stenciling or silk screening. The quantity of liquid carrier used in the mixme is selected to give the mixture the proper viscosity for the particular method employed in applying the mix- :ure to the base. After the layer has been applied to :he base, the base and layer are then fired to drive off the volatile carrier materials and to fuse the mixture Into a continuous phase of solidified glass with the metal particles dispersed uniformly throughout.

While the foregoing method results in extremely sat- .sfactory and useful resistance elements, it has been dis- :overed that the power stability of such elements and )ther characteristics thereof can be improved by introlucing and uniformly mixing with the metal-glass resistance composition certain filler materials in the amount )f about 3% to 40% by weight of the total mixture. )ne filler material preferably used for this purpose is ground fired steatite (MgSiO Other filler materials, illCll as alumina (A1 calcium silicate (CaSiO )arium silicate (BaSiO and lead zirconate (PbZrO nay also be employed with desirable results. The filler naterial need not consist entirely of any one of the above mentioned materials but may comprise suitable mixtures thereof.

The above-listed filler materials are compatible materials which, when mixed with a cermet material, are not totally dissolved in the glass or binder. The filler material is in powdered or finely divided form and mixed together with the soluble metal compound and glass powder to form a uniform mixture. While the addition of filler material to the mixture after the glass and noble metal solutions have been calcined does produce desirable increases in the resistance and does, to some extent, relieve the bleed-out eflect of the glass during the final firing step, it is believed essential, in order to obtain power stability, that the filler be added while the metal is still in the resinate or solution form. That is, it is believed necessary, to obtain improved power stability, to add the tiller material before the glass and noble metal solution are calcined. The entire mixture of glass, noble metal solution and filler material is then heated sufi'iciently to drive off the volatile organic and inorganic components and then calcined to remove any residual organics and to initiate whatever reaction occurs between the components.

The calcined material is then ground to the desired particle size which is preferably a size sufficiently small to pass through at least a 325 mesh screen. This powder may then be mixed with an appropriate carrier or medium to facilitate application of the resistance material to the substrate member so that it may be fired to a temperature sufficient to fuse the glass constituent and form a solidified glassy phase.

It has been found that the amount of filler which provides preferable and economically practicable results is in the range of about 5% to about 50% by weight of filler as a percentage of the cermet constituents Without the filler or, in the range of about 3% to 40% by weight of the total mixture including the filler.

The following example represents a typical high resistance resistor formed of cermet resistance material which has been found initially to be somewhat power unstable.

Example A.(Iridium-gold cermet without filler The initial resistance of the above resistor was 35.65K ohms per square. Power in the amount of .2 watt per .1 x .1 (or 200 milliwatts for a .1" X .1") was applied across the above resistor for 16 hours at an ambient temperature of C. After this period the resistance was again measured and found to be 34.84K ohms per square. The resistance change for this .1" x .1" resistor was .8K ohms or 800 ohms which represents a 2.24% change in resistance from the original measured resistance of the device. There are other mixtures of cermet resistance material having an initial resistance change amounting to greater than 5% of the original resistance.

It should be understood that the resistance material may sometimes be applied for a substantial length of greater than two or three inches and may amount to 20 or 30 square. The initial resistance change under power application for such a resistor can be substantial and may amount to 20,000 to 40,000 ohms in a resistor element of considerable length. In precise variable resistance devices and microcircuit technology, this amount of initial resistance change may be intolerable. It is desirable, therefore, to substantia ly reduce this initial resistance change due to power instability.

The following are examples of cermet mixtures formed in accordance with the present invention, illustrating the improved power stability characteristics of such resistance material.

Example B Percent by weight Glass No. 1 40.62 Glass No. 2 40.62

Iridium 10.41

Gold 3.47 Steatite (MgSiO 4.88

Initial Resistance 41.842K ohms per square. Power Operation 60 hours-O milliwatts per .1" x .1" square. Final Resistance 41.839K ohms per square. Resistance Change .003K ohms or 4 ohms. Percentage Resistance Change .007%.

Example C Percent by weight Glass No. 1 34.27 Glass No. 2 34.27 Iridium 9.08

Gold 3.02

Steatite (MgSiO 19.36

Initial Resistance 72.1K ohms per square. Power Operation 24 hours-200 milliwatts per .1" X .1". Final Resistance 7208K ohms per square. Resistance Change .02K ohms or 20 ohms. Percentage Resistance Change .03%.

Example D Percent by weight Glass No. 1 38.25 Glass NO. 2 38.25

Iridium 10.13

Gold 3.37 Steatite (MgSiO 10.00

Initial Resistance 40.980K ohms per square. Power Operation 60 hours-200 milliwatts per .1 x .1". Final Resistance 41.0l7K ohms per square. Resistance Change +.037K or 42 ohms. Percentage Resistance Change 092%.

Example E Percent by weight Glass No. 1 31.25 Glass No. 2 31.25

Iridium 8.28

Gold 2.75

Steatite 26.47

Initial Resistance 252.85K ohms. Power Operation 100 hours200 milliwatts per .1 x .1". Final Resistance 252.5K ohms per square. Resistance Change .35K ohms or 350 ohms. Percentage Change .l4%.

Example F Percent by Weight Glass No. 1 28.72

Glass No. 2 28.72

Iridium 7.61

Gold 2.52

Steatite 32.43

Initial Resistance 274.35K ohms per square. Power Operation hours200 milliwatts per .1 x .1. Final Resistance 272.25K ohms per square. Resistance Change -2.1K ohms or 2100 ohms. Percentage Change .77%.

Example G Percent by Weight Glass No. 1 38.21 Glass No. 2 38.21 Rhodium 2.46

Ruthenium 12.27

Iridium 4.09

Steatite 4.76

Initial Resistance 432.23 ohms. Power Operation hours milliwatts per .1" x .1. Final Resistance 431.88 ohms. Resistance Change .35 ohms. Percent Change .16%.

Example H Percent by weight Glass No. 1 33.44

Glass No. 2 33.44

Rhodium 2.15

Ruthenium 10.72

Iridium 3.58

.Steatite 16.67

Initial Resistance 706.37 ohms per square. Power Operation 100 hours120 milliwatts per .1" x .1". Final Resistance 706.33 ohms per square. Resistance Change .04 ohms. Percentage Change .0l%.

In all of the above examples the power applied was 120 or 200 milliwatts to a square of resistance material .1 by .1" square. The ambient temperature during the test was 70 C. and all of the resistance elements had a temperature coefiicient of resistance which was generally within the commercially accept-able range of $300 p.p.m./ C.

It is believed that the particular composition of glass is not critical to the practice of the invention, except that it must have a melting temperature below that of the metal constituents. The following are illustrative examples of the composition of the glasses employed in the above examples.

Glass N0. 2

Glass No. 1

:ration of metals during the firing of cermet resistance materials which tends to create a somewhat uneven disribution of the metal particles within the resistor. The tddltlOIl of a filler material into the metal-glass mixture, vhile the metal is still in the resinate form (or in the 'orm of a metal solution), appears to inbihit the agglomtration of certain metal alloys during the calcining and using operations.

The addition of fillers, and particularly calcined steaite, are particularly effective when employed with noble netal alloys containing at least one of the noble metals ridium, rhodium and ruthenium. It is believed that durng the calcining process the metal particles are attached iot only to the glass particles but to the filler particles. It is believed that the metal particles in resinate form, )1 in the form of a metal solution, become attached to roth the glass particles and the particles of filler material. ['hese metal particles are extremely small and are physi- :ally coated to the glass and filler material particles so hat the metal alloy is very uniformly distributed through- )ut the mixture. During the fusing step, it is believed that he filler partially dissolves in the glass and the metal )articles attached thereto are more evenly distributed hroughout the solidified resistance material and form nany more high resistance conductive paths through the 'esistor. It is believed that the more evenly distributed the netal particles are within the hinder, the less structural :hange is likely to occur when power is applied to the 'esistor. It is also very likely that the presence of the iller material also physically obstructs any changes or 'eorganization of the metal particles within the solidified naterial upon the application of power thereto.

The filler materials are not completely dissolved in he glass, during the firing or fusing step, and this apparzntly increases the viscosity of the glass during the firing )peration. Thus, the partially soluble filler materials func- :ion to control the flow or bleed-out of the glass when he cermet is fired.

The addition of filler material, and especially steatite, o cermet resistance materials also greatly reduces the indesirable reaction between the cermet glasses and the luxes found in most presently employed end termination naterials and the cermet resistance films. As mentioned )reviously, these fluxes, which are to some extent soluble n the binders or glasses employed in cermet materials, :reate a reaction when applied to an unfilled cermet re- :istor and the result is a high resistance terminal connec- :ion. The addition of a filler, such as steatite, to the re- ;istance material tends to prevent these fluxes from 'eacting too deeply into the cermet film during firing of he termination. The resistance of the termination is 'educed and the quality of such termination is greatly mproved so that much higher yields may be accomalished.

While in accordance with the patent statutes, repreaentative examples have been submitted and there have )8611 described what at present are considered to be the )referred embodiments of the invention, it will be under- ;tood that various changes and modifications may be nade therein without departing from the invention and t is, therefore, the aim of the appended claims to cover ill such changes and modifications as fall within the true tpll'lt and scope of the invention.

What is claimed is:

1. A method of forming an electrical resistance element having high power stability including the steps of:

forming a mixture of finely ground particles of glass,

a solution of dissolved noble metal compound including at least one of the noble metals selected from the group consisting of iridium, rhodium and ruthenium, and particles of at least one filler material selected from the group consisting of MgSiO A1 0 CaSiO BaSiO PbTiO and PbZrO heating said mixture to drive off the volatile materials thereby producing a dry mixture of glass, filler material and finely divided particles of a noble metal alloy;

grinding the dry mixture to a powder;

mixing the dry powder with a volatile liquid to form a viscous mixture;

applying a layer of the viscous mixture to a high temperature resistant, electrically non-conductive substrate;

heating the substrate and layer to at least the melting temperature of the glass constituent but less than themelting temperature of the metal alloy to produce a continuous glassy phase having the metal alloy particles and filler material uniformly dispersed therethrough.

2. A method of forming an electrical resistance element having high power stability including the steps of:

forming a mixture of finely ground particles of glass, a solution of dissolved noble metal compound formed of at least one of the noble metals selected from the group consisting of iridium, ruthenium and rhodium and at least one of the metals selected from the group consisting of gold, palladium, silver and platinum, and particles of at least one filler material selected from the group consisting of MgSiO A1 0 CaSiO BaSiO PbTiO and PbZrO heating said mixture to drive off the volatile materials thereby producing a dry mixture of glass, filler material and finely divided particles of a noble metal alloy;

grinding the dry mixture to a powder;

mixing the dry powder with a volatile liquid to form a viscous mixture;

applying a layer of the viscous mixture to a high temperature resistant, electrically non-conductive substrate;

heating the substrate and layer to at least the melting temperature of the glass constituent but less than the melting temperature of the metal alloy to produce a continuous glassy phase having the metal alloy particles and filler material uniformly dispersed therethrough.

References Cited UNITED STATES PATENTS 1,764,311 6/1930 Hunt 252-508 2,718,577 9/1955 Sherk 20167 2,786,819 3/1957 Smith et al 252-519 2,950,996 8/1960 Place et al. 117-227 LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

US. Cl. XJR. 

